1. Field of the Invention
The present invention relates to a monitor photo diode (PD) of an optical pickup system, and more particularly, to an apparatus for and a method of controlling a monitor PD that enhances the performance of the monitor PD by controlling a dynamic range of the monitor PD.
2. Description of the Related Art
An optical pickup system is generally used to write data on or read data from an optical disk. In particular, such an optical pickup system uses light emitted from a laser diode (LD) and having a predetermined power to write data on, read data from, or erase data from the optical disk. A monitor photo diode (PD) or a front PD (FPD) monitors light power emitted from the LD and feeds back monitoring results to an optical power controller. The optical power controller adjusts the power of the LD based upon optical power information derived from the monitoring results fed back thereto from the monitor PD.
FIG. 1 is a block diagram of an optical pickup system. Referring to FIG. 1, the optical pickup system includes an LD 100, an optical device 110, a monitor PD 120, and a controller 130. In order to write data on or read data from an optical disk 140, the controller 130 transmits an electrical signal to the LD 100. The LD 100 outputs an optical signal corresponding to the power of the received electrical signal. Most of the output optical signal, for example, 90%, is emitted toward the optical disk 140 via the optical device 110. A remainder of the output optical signal, in this example, 10%, is input to the monitor PD 120. The monitor PD 120 converts the optical signal received via the optical device 110 into an electrical signal and feeds back the electrical signal to the controller 130. The controller 130 adjusts the power of the electrical signal transmitted to the LD 100 based on the electrical signal fed back from the monitor PD 120. This process is called automatic power control (APC).
FIGS. 2A and 2B are diagrams illustrating an optical power signal transmitted to the LD 100 and a signal output from the monitor PD 120 in response to the optical power signal when carrying out a data writing process. When writing data on the optical disk 140, the LD 100 is overpowered with a highest level of power so as to be preheated. Thereafter, the LD 100 is given a write power, which is lower than the preheating power, and then a read power, which is lower than the write power. The monitor PD 120 outputs a signal having a predetermined gain and corresponding to the power applied to the LD 100 at any given time.
However, when the temperature of the LD 100 increases, the efficiency of outputting light generally decreases. Therefore, even though the optical pickup controller 130 commands that the LD 100 emit an optical signal having a constant level of intensity, an optical signal output from the LD 100 may have a lower intensity than expected due to the increase in the temperature of the LD 100. This is why the optical power controller 130 performs the APC process described above by making the monitor PD 120 keep monitoring optical power output to the LD 100 and feed back the monitoring results.
FIG. 3 is a graph illustrating a dynamic range of the monitor PD 120. When power applied to the LD 100 reaches a maximum, the output of a monitor PD 120 is expected to be a minimum. On the other hand, when the power applied to the LD 100 reaches a minimum, the output of the monitor PD 120 is supposed to be a maximum. The maximum power of the monitor PD 120 is a predetermined reference voltage Vref, and the minimum power of the monitor PD 120 is a predetermined cut-off voltage. The cut-off voltage of the monitor PD 120 (1V in general) is generally set in consideration of the maximum power (overpower in general) applied to the LD 100. Therefore, when the reference voltage Vref and the cut-off voltage are maintained at a predetermined level, the gradient of the graph of FIG. 3 varies depending on the type of an optical disk 140. For instance, the gradient is affected by the density of the optical disk 140, and data writing or reading speed, which are factors that directly affect the maximum power applied to the LD 100. In other words, when reading data from optical disks 140 having different data writing/reading speeds and different writing densities using an optical pickup system to which a fixed reference voltage is applied, the dynamic range of the monitor PD 120 of the optical pickup system varies.
For example, in the case of an optical disk 140 that requires a relatively higher optical power, the output of a monitor PD 120 varies considerably less depending on optical power. In this case, the dynamic range of the monitor PD 120 is lessened in response to variations in power output to an LD 100, thus degrading the precision of APC carried out by the optical pickup system.